1. Field of the Invention
The invention relates generally to electromagnetic actuators. More particularly, the invention relates to an actuator that produces linear movement. Although not limited thereto, devices in accordance with the present invention are especially useful in positioning a magnetic transducer or print head relative to a disk in a disk memory system or the like.
2. Prior Art
The use of linear actuators or linear motors to position a transducer relative to a selected disk in a disk memory system is well known in the prior art. Prior art linear actuators consist of a frame to which a magnetic structure is attached. The magnetic structure generates a plurality of magnetic flux lines. A coil is positioned within the magnetic structure and is subjected to the magnetic flux line. As is well known in the art, by passing current through the coil, a force is created which propels the coil in its to and fro motion. Attached to the coil is a carriage assembly to which the head is attached so as to access (that is read or write) data on a selected disk in the disk pack.
In order to restrict the motion of the coil and its carriage assembly to a linear path, an elongated precision rod is mounted to the frame and relative to the magnetic structure. The precision rod is generally aligned in precise parallelism with the desired path of movement of the magnetic transducer. Sleeve bearings are used singly or in combination with other forms of sliders to propel the carriage assembly along the precision rod. Invariably the rod is supported at its two extremeties only, which result in unusual flexing of the rod as the carriage is transported to and fro. A more detailed discussion of the structure of the prior art linear actuators may be found in U.S. Pat. Nos. 3,587,075 and 3,899,699.
Although the prior art linear actuators work satisfactorily for its intended purpose, these actuators are plagued by several drawbacks. Before addressing the drawbacks of the prior art actuators, it is worthwhile noting that in order to control the accessing of data from a disk pack the carriage assembly is controlled by a servo loop. As is well known to those skilled in the art, the servo loop for a linear actuator is functional or effective over a given frequency range. The lower end of the frequency range is called the cross-over point. The frequency range in turn is related to the track density of the recorded data. The trend in present day disk storage systems is to provide high performance storage systems. High performance storage system means a storage system in which the track density is relatively high, for example, in the range of 100 tracks per inch.
Since the functional frequency range for the controlling servo loop is inter-related with data density, the higher the data density the higher is the functional frequency range for the associated controlling servo. Alternately, a storage system in which the functional frequency range of the associated servo is narrow unnecessarily restricts the data density of the system.
In view of the inter-relationship between data density and the functional frequency range of the controlling loop the optimum condition is to have a servo loop which is effective to control over a relatively wide frequency range.
One factor which adversely affects the controlling servo (which may be closed loop) of a linear actuator is the resonant frequencies of the actuator. Particularly, the resonant frequency affects the functional frequency range of the servo. The resonance in the actuator is transferred to the transducer which rides on the carriage of the actuator. Because the transducer is in the controlling servo loop, an instability is introduced into the controlling servo loop. The net result is that the servo cannot control the movable assembly so that the transducer can faithfully follow a selected track on a target disk.
Whenever the resonant frequency is relatively close to the functional frequency range of the controlling servo, a plurality of servo errors is generated. The errors adversely affect system throughput and system reliability. The resonant frequency is a direct result of mechanical vibration in the actuator. Although all actuators will vibrate at some frequency, the desirable approach is to design the actuator so that it will resonate at a relatively high frequency so that the resonant frequency of the actuator does not affect the functional frequency range of the controlling servo.
In order to maintain system reliability, if the linear actuator has a relatively low resonance frequency, then the functional frequency range for the controlling servo is invariably forced to be lower than the resonant frequency of the system. In view of the above discussion, this condition implies a low density storage disk system, an undesirable result.
Returning now to problems affecting prior art actuators, and in particular, actuators for use with flexible disk storage systems perhaps the most pressing problem is that these actuators have a relatively low natural resonant frequency; typically from 100 to 500 hertz. Due to the low resonant frequency response, the previously described defects which are associated with linear actuators having low resonant frequency response are attributes of the prior art actuators. This being the case, the prior art actuators are unsuitable for use in high density flexible disk storage systems.
One of the contributing factors for the low frequency response of the prior art actuator is the fact that the precision rod which guides the carriage assembly is susceptible to unusual flexing. As stated previously, the precision rod is only supported at its two ends with no support along its length.
Another factor stems from the fact that the slider side which rides against the precision rod creates an unusual amount of frictional resistance to motion.
Another problem which affects the prior art actuator is that the actuators do not lend themselves to modular design. One important characteristic of a modular design is that functional elements (e.g., the carriage assembly, etc.) hereafter called Field Replaceable Unit (FRU), can be changed in an actuator without interrupting the actuator's alignment with its associated disk storage system.
The non-modularity characteristic of prior art actuators stems from the fact that the design philosophy in these actuators requires the center of thrust or center of motive force (supplied by the coil) must coincide with the center of mass of the carriage assembly. This design philosophy requires a more complicated design which does not lend itself to modularity.
Associated with the non-modularity defects of the prior art is the further defect that the prior art actuators cannot be satisfactorily arranged so that a plurality of these actuators access a common disk storage system. One of the restraints which is necessary for plural accessing is that the separation be minimum between the carriage assembly, including the head arm with transducer thereon. To satisfy this restraint, it is necessary that at least one side of the actuator, preferably next to the carriage assembly and along the direction of actuator stroke, be a reference side, preferably flat. This appears to be the most auspicious method of designing the actuator so that a second actuator with a characteristic side similar to the previously described side can be placed adjacent to each other without unnecessary interference with one another. However, due to the complicated design of prior art actuators, the minimum separation requirement cannot be realized and, therefore, said actuators cannot be used for plural accessing.
Still another problem which is associated with the prior art actuators is that the coil which produces the motive force for moving the carriage assembly is not self supporting. A self supporting coil is one which does not require a bobbin to support it when it is used in a linear actuator. Almost invariably, the coil used in prior art actuators are wound on a coil supporting member generally called a bobbin. The bobbin and coil are then positioned within the air gap formed by the actuator's structure and are used to position the head assembly. Several undesirable results are associated with these coil bobbin assemblies. Firstly, the mass of the moveable assembly is increased. With more mass, more current is needed for driving the actuator. More current increases the cooling requirements and cost of the actuator. Probably more important is the fact that the bobbin tends to reduce the natural resonance frequency of the actuator and, as stated previously, adversely affects the overall operation of the actuator.